Mesophyll conductance in Zea mays responds transiently to CO 2 availability: implications for transpiration efficiency in C 4 crops

Summary Mesophyll conductance ( g m ) describes the movement of CO 2 from the intercellular air spaces below the stomata to the site of initial carboxylation in the mesophyll. In contrast with C 3 ‐ g m , little is currently known about the intraspecific variation in C 4 ‐ g m or its responsiveness to environmental stimuli. To address these questions, g m was measured on five maize ( Zea mays ) lines in response to CO 2 , employing three different estimates of g m . Each of the methods indicated a significant response of g m to CO 2 . Estimates of g m were similar between methods at ambient and higher CO 2 , but diverged significantly at low partial pressures of CO 2 . These differences are probably driven by incomplete chemical and isotopic equilibrium between CO 2 and bicarbonate under these conditions. Carbonic anhydrase and phosphoenolpyruvate carboxylase in vitro activity varied significantly despite similar values of g m and leaf anatomical traits. These results provide strong support for a CO 2 response of g m in Z. mays , and indicate that g m in maize is probably driven by anatomical constraints rather than by biochemical limitations. The CO 2 response of g m indicates a potential role for facilitated diffusion in C 4 ‐ g m . These results also suggest that water‐use efficiency could be enhanced in C 4 species by targeting g m .